1. Field of the Invention
This invention relates to an electrostatic recording head used in an electrostatic recording apparatus, such as, an electrostatic plotter and also to a method of driving the same.
2. Description of the Related Art
A prior art electrostatic recording head includes a multiplicity of recording electrodes, for example, needle-like electrodes linearly arranged in one or two lines in a direction substantially perpendicular to the direction of feeding a recording medium, and a plurality of auxiliary electrodes disposed in the vicinity of the needle-like electrodes. The needle-like electrodes are divided into a plurality of groups, and the plurality of auxiliary electrodes are disposed such that the needle-like electrodes belonging to each group are located to face one or two auxiliary electrodes. The term "face" used herein is intended to include the case where both the recording electrode and the auxiliary electrode are located opposite to each other on the side of the recording surface of the recording medium or the case where these electrodes are located opposite to each other on both sides of the recording medium, respectively.
A method using this electrostatic recording head for forming an electrostatic latent image of an image to be recorded on a recording medium is well known and will now be briefly described. The image to be recorded is scanned with lines one by one to generate a binary printing signal of a high level or a low level according to the brightness of each of the picture elements on one scanning line of the image being scanned. The needle-like electrodes of the electrostatic recording head are provided in the number corresponding to the number of the picture elements on one scanning line of the image. A predetermined voltage of, for example, -300 V is applied to the needle-like electrode in each group when the printing signal generated from the corresponding picture element is at the high level, and at the same time, another predetermined voltage of, for example, +300 V is applied to the auxiliary electrode facing that needle-like electrode. As a result, discharge occurs between the needle-like electrode and the auxiliary electrode through the recording medium, and an electrostatic latent image corresponding to the printing signal is formed on the recording medium. On the other hand, when the printing signal is at the low level, the above voltages are not applied to the needle-like electrode and the auxiliary electrode, and no discharge occurs between them, with the result that no electrostatic latent image is formed. In this manner, the predetermined voltage is selectively applied to each of the needle-like electrodes belonging to one group according to the level of the corresponding printing signal. Similar processing is successively executed for the electrodes of all the groups until the electrostatic latent image of the part of the image corresponding to one scanning line is formed. By the repetition of the above manner of processing in which the image is scanned with lines one by one while feeding the recording medium in a predetermined direction, the electrostatic latent image of the image is formed on the recording medium. The brightness of the image is determined by the density of the printing signals of the high level. Such an electrostatic recording head is disclosed in, for example, JP-A-3-197987, copending U.S. patent application Ser. No. 08/021,559, filed on Feb. 24, 1993 entitled "AN ELECTRODE DRIVING DEVICE IN ELECTROSTATIC RECORDING APPARATUS". The recording medium commonly includes a laminate of a base layer of an electrical insulating material, such as, paper or plastics, a layer of an electrical conductive material and a layer of a dielectric material.
In the case of the electrostatic recording head of the above type, it is known that striations corresponding to the respective groups of the needle-like electrodes appear in parallel to the direction of feeding the recording medium when an image, for example, a totally black image is to be formed. This is attributable to various causes. One cause is that, the application of the voltage to the auxiliary electrode induces polarization in the dielectric layer of the recording medium, and when the voltage is applied to the auxiliary electrode again before the polarization is released, this leads to a non-uniform spatial distribution of the electric field. According to another cause, a large capacity is induced at either end of each group of the needle-like electrodes due to the fact that the needle-like electrodes are closely arranged in each group. Still another cause is that, when the voltage is applied to almost all the needle-like electrodes belonging to the group, the spatial distribution of the electric field tends to become quite non-uniform at either end of the group due to the edge effect.
Therefore, various methods of driving the electrostatic recording head are proposed. For example, in one of the head driving methods, the discharge order is so selected that, after one group effects the discharge, another group on the head remote as far as possible from the one group is selected as the next one that effects the discharge, or the individual groups are successively driven to effect the discharge according to such a discharge order that the time interval between the discharge of one group and the next discharge of the group adjacent to the one group is made as long as possible. In another head driving method, the boundary between the groups is changed each time an electrostatic latent image representing one line of an image is formed. However, undesirable striations tend to appear on a recording medium due to another cause although the adverse effect of the polarization induced in the dielectric layer of the recording medium can be eliminated by the former method. Also in the case of the latter method, appearance of undesirable striations cannot be so effectively minimized. Therefore, in the case of the prior art electrostatic recording head of the multiplex type, it is very important to minimize appearance of such undesirable striations.
U.S. patent application Ser. No. 08/021,559 cited above proposes a method of solving such a prior art problem as pointed out above. This U.S. patent application is incorporated herein by reference.